Our world is bathed in electrical noise. Electrical noise can make it difficult to properly measure or transmit small electrical signals by causing electrical interference signals to be present along with the electrical signals of interest. Signals detected with high impedance sensors and/or processed by high gain amplifiers are particularly prone to being contaminated by electrical interference. An extremely wide range of devices used in audio, engineering, medicine and research incorporate high impedance electrical sensors. Examples include microphones, guitar pickups, skin sensors (EKG, EMG, EEG) and microelectrodes used in the neurosciences.
Electrical noise or "interference" is typically the result of the electromagnetic fields generated by electrical currents flowing in electrical or electronic circuits and equipment. Electromagnetic fields can cause interference by several mechanisms. High impedance electronic circuits and ground loops may act as antennas and pick up the electromagnetic field. In other cases poor ground lines carrying high currents can cause ground signals to develop a periodic voltage fluctuation.
Many of the most common and troublesome sources of electrical noise are periodic. The most common form of electrical interference is generated by the power mains but there are other sources. For example, the screen refresh on a computer monitor or television can generate periodic electrical interference. Electrical interference may be induced by alternating current in power supplies, lights and/or other electrical equipment. Periodic electrical interference appears as a repeating waveform superimposed on the signal of interest. The period of the interference signal is the same as the period of the source of the interference. The electrical interference signal may have a very complicated waveform. For example, the wave form of an electrical interference signal caused by power mains in North America would usually appear when viewed on an oscilloscope as a complex spike which repeats at a rate of 60 Hz.
Electrical interference is a major concern in scientific research and in the music industry since it can significantly degrade signal quality. Unfortunately, it is notoriously difficult to remove without also altering the original signal of interest imbedded in the noise. What is needed, and what is not provided in the prior art, is a good method for removing periodic electrical interference from an electrical signal without significantly affecting the electrical signal.
The most common method for removing periodic electrical interference from a signal is to pass the signal through a notch filter tuned to the fundamental frequency of the electrical interference. A notch filter is designed to remove all signal components at a specific frequency and to let all other frequencies pass without attenuation. For example, for removing electrical interference induced by the power mains in North America a notch filter tuned to 60 Hz would be used because 60 Hz is the fundamental frequency of the a.c. power supplied in North American power mains (power mains operate at a frequency of 50 Hz in many other regions). This method effectively eliminates the 60 Hz component of the electrical interference. Unfortunately, it also eliminates the 60 Hz components of the primary signal of interest which also pass through the filter. This loss of signal components, and the introduction of phase distortion, makes the use of notch filters unacceptable when precise characteristics of the signal of interest are important.
Another problem with using a simple notch filter to remove electrical interference is that a notch filter often fails to remove all of the electrical interference even if the filter is accurately tuned to the fundamental period of the electrical interference signal. This is because interference usually has significant power at various harmonics of its fundamental frequency. For example, the electrical interference induced by fluorescent lamps often has frequency components at several harmonics of 60 Hz (120 Hz, 180 Hz, 240 Hz . . . n*60 Hz). Some of these harmonics have frequencies greater than several kHz and many harmonics may have more power than the fundamental. A simple notch filter tuned to 60 Hz will only eliminate the fundamental while the harmonics continue to contaminate the signal.
Comb filters are filters which remove signal components at a fundamental frequency and also remove signal components at harmonics (i.e. integer multiples) of the fundamental frequency. In some applications comb filters may be used to eliminate electrical interference from a signal. However, comb filters will also eliminate the frequency components from the signal of interest at each of the multiple corner frequencies of the comb filter. As a result, the signal of interest is distorted.
Some have attempted to reduce the effect of notch or comb filters on a signal of interest by using notch or comb filters with very narrow notches. Notch filters produce maximal attenuation of the signal at the notch frequency (e.g. 60 Hz). However, they also attenuate frequency components which are slightly above of below the notch setting. The width of the notch refers to the range of frequencies influenced by the filter. If a very narrow notch is used then fewer frequencies in the signal of interest are affected by the filter. However, the narrower the notch, the greater the need for precise matching of the center frequency of the filter to the periodicity of the noise generator. Furthermore, narrow band filters tend to oscillate in response to transient events such as a pulse or the edge of a square wave signal.
The problem of ensuring that the center frequency of a very narrow notch filter is tuned to the frequency of a noise signal may be addressed by using adaptive filters. Adaptive filters monitor a reference signal from the source of the electrical interference (the "noise generator") and use this information to adjust the frequency of the filter to coincide with the precise frequency of the noise. Although use of a finely tuned notch filter decreases distortion of the signal of interest, distortion remains a problem. Furthermore, the need for comb filters in many applications leads to potential distortion of the signal of interest at frequencies equivalent to the fundamental and each of the harmonics of the noise generator.
Removing an electrical interference signal without affecting a signal of interest is a particular problem where the signal of interest is music or other audio signals. Musical signals normally contain a wide range of frequency components spanning the complete bandwidth of the recording system. For example, audio signals are made up of frequencies ranging roughly from 20 Hz to 20 kHz. These frequencies overlap with the 60 Hz fundamental frequency of power mains generated electrical interference as well as all harmonics of 60 Hz up to the 20 kHz upper limit of the audio signal. There are more than 300 evenly spaced harmonics across this range. A comb filter would need to have notches at each of these harmonics to remove all of an electrical interference signal having a fundamental frequency of 60 Hz. Although such a comb filter would eliminate the electrical interference signal, it would also remove all components of the audio signal with frequencies equal to 60 Hz, 120 Hz, 180 Hz . . . 19,920 Hz, and 19,980 Hz. The distortion caused by filtering these frequencies from the audio signal usually makes this approach unacceptable. Signals recorded in clinical medicine, medical research, biological research and other scientific and engineering activities also have frequency components which span the same range as the fundamental and harmonics of periodic electrical interference. Therefore, notch filters are also unacceptable for noise reduction in most of these applications.
Kobatake, Synchronous Adaptive Filter, Papers of Society of Instrument and Control Engineers, v. 19, No. 3, pp. 34-40 discloses an adaptive noise canceller for periodic interference. Kobitake notes that his system acts as a series of notch filters at harmonics of the fundamental frequency and notes that his system could cause distortion of a signal of interest at higher harmonics of the fundamental frequency of the filter.
Nakajima et al. U.S. Pat. No. 5,029,118 describes a system and method for cancelling a periodic noise superimposed on a real signal component of an original signal. The primary problem addressed by Nakajima et al. is to cancel a relatively high frequency periodic noise signal from a signal containing a relatively very slowly varying d.c. signal. Nakajima et al. discuss applications in detecting the torque on the output shaft of a torque converter in an automotive automatic transmission.